The transfer of electrical power from a primary to a secondary winding of a transformer is a well known task. However, when the primary and secondary windings are separated by an air gap, and when the available power is limited, it is extremely important to maximize power transfer efficency. Such a situation is encountered in connection with Smart Cards having an air-gap (contactless) interface. In one early design, power was transferred across a contactless interface via compacitive coupling. U.S. Pat. No. 4,480,178 discloses a tuning arrangement designed to enhance such coupling. Nevertheless, greater efficiency was needed, so transformer coupling was used to increase transfer efficiency. U.S. Pat. No, 4,692,604 teaches the use of a flexible magnetic but non-magnetostrictive core piece for use in connection with an inductor acting as the secondary portion of a power transformer. While significant improvement is achieved, only a limited amount of magnetic material can be used in the available space, and situations arise where only a small battery is available to provide power across the contactless interface.
It is therefore an object of the present invention to improve power transfer efficiency of a transformer whose primary and secondary windings are separated by an air-gap.
It is another object of the present invention to achieve efficient power transfer from a low voltage source.